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Hypertension: A Case Study

Gene Discovery Reports in the Popular Press and Research Journals

Currently
Under Consturction

What exactly is high blood pressure?

High blood pressure is the common name for the medical condition
known as essential hypertension (I will hereafter use the term hypertension
when refering to the disease). Hypertesion has received a great deal of attention
in biomedical research because it affects a large portion of the adult population
worldwide. In the United States, the National Center for Health Statistics estimates
that some 25% of the adult popultion is hypertensive. The incidence of hypertension
is even greater in the African-American population.

Once a patient's blood pressure consistenly exceeds 140 mmHg (systolic)
and 90 mmHg (diastolic), he or she is diaganosed with hypertension. By contrast
the accepted normal heart rate for the adult popultion is 120/80 mmHg. The numerator
represents the systolic blood pressure, and the denominator represents the diastolic
blood pressure. When the heart contracts, the instaneously surge of fluid (blood
in this case) exerets a pressure on the arterial walls. The systolic pressure
reflects the stress placed on arterial walls during contractions, and the diastolic
pressure value denotes the pressure between contractions, when the arterial
walls are relaxed.

As one might expect increased blood pressure poses a health risk
over time. In addition to subjecting the circulatory system to greater stress,
hypertension means that a patient's heart works harder than normal. As a result,
hypertension greatly increases the chance that a patient develops kidney damage,
loss of vision, atherosclerosis, cardiac arrest, or stroke. Given the risks
associated with the disease, medical researchers have long worked to elucidate
the molecular cause of blood pressure. Naturally geneticist posited that hypertension
may also be due to genetic mutations, and so began the search.

Does a hypertension gene exist?

Since 1990 biomedical researchers have announced on several occasions
that the deffective gene in persons with hypertension had been found. How can
it be? Any observer will note that either several genes may be involved in hypertension
or scientists are mistaken.

For many of us our first encounter with genetics teaches us the
single gene-single trait model. I call this view Mendelian thinking. It follows
from the Mendelian perspective that a disease should be the result of a defective
protein, which in turn was due to a defective gene. Thus, in the case of hypertension,
one would expect an altered gene to produce an abnormal protein that was responsible
for the disease. Below I examine attempts to identify the genetic causes of
hypertension.

Does hypertension fit the Mendelian model?

AGT1 gene

In October 1992 the New York Times printed an article
that announced the discovery of a gene linked to high blood pressure (more
specifically, essential hypertension). The scientists responsible for the
discovery told the journalist that they suspected an association hypertension
and the angiostensinogen (AGT) gene. Although heralded as the first to link
hypertension to a gene, the researchers could not prove definitively that
mutations in the AGT1 gene caused hypertension. Other geneticists interviewed
in the article claimed the finding was encouraging because it finally dispelled
the notion that it would be impossible to identify a single gene associated
with hypertension.

The corresponding scientific paper published in the journal
Cell had the following title, "Molecular basis of human hypertension:
role of angiostensiongen." A team lead by Jean-Marc Lalouel at the University
of Utah authored the study. Lalouel and collaborators reported that they found
a correlation between allelic AGT polymorphisms and the affected phenotype.
That is to say, hypertensives (i.e., patients with hypertension) generally
had a specific variant of the AGT protein that consitently correlated with
the disease. Thus they concluded that certain inherited genetic variants of
AGT predispose persons to hypertension (Jeunemaitre, et.al. 1992).

Conclusion

It may be objected that a conclusion concerning the genetic cause
of hypertension cannot be rendered based upon data that is more than ten years
old. That objection is quite true. The above two examples were included to examine
the manner in which gene discoveries are reported in scientific journals and
in the lay press. In addition an investigation of the more recent hypertension
studies would be incomplete without an understanding of the early attempts the
isolate a gene, inasmuch as many of the most recent studies are also linkage
experiments. Finally the 1992 announcement is an important starting point in
that much of the reporting on hypertension becomes sparse in the decade that
follows. The term sparse is used to emphasize that the reports of advancements
in the search for the genetic causes of hypertension did not match the large
number of papers published in the scientific community since 1992 (of which
there are many).

Both the Times article and the Cell paper leave
the reader with a sense that identification of the AGT gene has opened more
future avenues of research, rather than providing all the answers to hypertension.
A discussion of future implications is pro forma is the scientific
community, but the Times articles exceeds expectations in that regard.
The Times journalist includes a discussion of the less clear inferences
of the study, namely the extent of environmental influence in producing the
disease and the frequency of similar AGT variations in the population at large.
Whereas the title of the Times article may have conveyed a sense that
scientists had uncovered the mysteries of hypertension, a careful reading of
the text provides a more accurate depection of thinking in the medical community.
That said, the newspaper title remains more affirmative than the journal title.
That latter suggest that AGT is merely one of many proteins that has role in
hypertension.